The present invention relates to a housing which enables radiation detectors to be accommodated within cassette cases in a reliable manner when they are used in a positron computed tomographic (CT) apparatus or a single-photon emission CT apparatus.
FIG. 5 is a schematic representation of a recently proposed positron CT apparatus capable of high resolution. As shown, this apparatus comprises basically a ring-shaped gantry 1 and a holder 3 such as a bed which holds the subject 2 in position.
The gantry 1 comprises a plurality of generally fan-shaped cassettes 4 for detecting gamma-rays that are arranged in annular form. Each of these cassettes accommodates four rows of a radiation detector 14 consisting of a scintillator 5 and a photomultiplier tube 6. These cassettes are juxtaposed in five stacks and a shield collimator 7 is provided on both the inside and the outside of the five stacks of cassettes 4.
Further, a slice collimator (not shown) is provided as required between adjacent stacks of scintillator 5 to reduce the entrance of background noise (i.e., scattering radiation and single gamma-rays).
Each of the radiation detecting cassettes 4 is accommodated in a cassette case 8 as shown in FIG. 4. This cassette case 8 is made of a material such as aluminum or a plastic resin that is transmissive of radiation (gamma-rays) but which is opaque to light. The cassette case 8 is provided with spaced partitions 9 that are formed as integral parts of this case. A radiation detector 14 consisting of a scintillator 5 and a photomultiplier tube 6 and which is fixed by a bleeder socket 12 is accommodated in each of grooves 10 which are delineated by side walls 11 and adjacent partitions 9. A circuit board 13 containing a bleeder circuit and any other necessary circuits is provided in the cassette case 8 along its outer circumference.
The photomultiplier tube 6 in each radiation detector 14 is usually encased within a glass tube having variations in its outside dimensions. Hence, each of the grooves 10 in the cassette case 8 is made considerably larger than the dimensions of the glass tube. However, this creates an unavoidable gap between the glass tube and each accommodating groove 10 and on account of nonuniformity in the size of this gap, the photomultiplier tube 6 and scintillator 5 of which each radiation detector 14 is composed will rattle or move about to make it impossible to perform precise measurements. Further, the rattling photomultiplier tube or scintillator can damage themselves. In order to avoid this problem, one may measure the nonuniform gap and insert a spacer of commensurate size. However, because of the thermal expansion mismatch between the aluminum cassette case 8 and the glass tube the radiation detector 14 might be damaged resulting from a temperature elevation that occurs after the spacer is fitted into the gap. Further, the gap must be covered with an aluminum foil or some other light-shielding material in order to avoid the noise problem due to entrance of ambient light.